In recent years, in-cylinder or direct injection type internal engines have been developed as internal combustion engines for motor vehicles. In this type of in-cylinder injection type engine, fuel is directly injected into a cylinder, or a combustion chamber, of the engine, and therefore various devices have been employed for forming a fuel-air mixture having an air fuel ratio close to the stoichiometric ratio, only in the vicinity of a spark plug provided in the cylinder. In the in-cylinder injection type engine equipped with such a device, the fuel can be fired or ignited without fail and burned well, even if the fuel-air mixture in the cylinder as a whole is a lean fuel mixture, namely, even if the average air fuel ratio is greater than the stoichiometric ratio. As a result, carbon monoxide (CO) or hydrocarbon (HC) contained in exhaust gases from the engine can be reduced, and the amount of consumption of the fuel can be significantly reduced during an idling operation of the engine, or during a steady-state running of the vehicle on which the engine is installed. In the conventional type of internal combustion engine wherein the fuel is injected into an intake passage, the fuel-air mixture is produced inside the intake passage, thus causing a delay with which the mixture actually flows into the cylinder. In contrast, the in-cylinder injection type engine, as described above, is free from such a delay, and may be accelerated or decelerated with excellent response.
However, the advantages of the in-cylinder injection type engine, as described above, can be provided only in the case where the engine is operated with a relatively low load applied thereto. Namely, if the amount of injection of the fuel is increased with an increase in the load of the engine, the fuel-air mixture formed around the spark plug tends to be excessively rich, namely, contains an excessively high percentage of fuel, thereby making it impossible to ignite the fuel, with a result of a failure in burning the fuel. Thus, in the case of the in-cylinder injection type engine, it is difficult to form a fuel-air mixture having the optimum air-fuel ratio only in the vicinity of the spark plug, over the entire range of operating states of the engine.
To solve the above-described problem, an in-cylinder injection type engine, as disclosed in laid-open Japanese Patent Publication (kokai) No. 5-79370, has two fuel injection modes, i.e., an early injection mode in which the fuel is injected during a suction stroke, and a late injection mode in which the fuel is injected during a compression stroke. This type of engine is controlled so that the fuel injection mode is selected from the early injection mode and the late injection mode, depending upon the load of the engine. When the fuel is injected in the late injection mode, a fuel-air mixture having an air-fuel ratio that is close to the stoichiometric ratio is formed only in the vicinity of the spark plug. Therefore, the fuel can be fired or ignited even when the mixture in the cylinder as a whole is a lean fuel mixture, and the amounts of CO and HC in the exhaust gases can be reduced. Further, the amount of consumption of the fuel can be greatly reduced during idling operations of the engine or steady-state running condition of the vehicle. When the early injection mode is established, on the other hand, the fuel is injected into a cylinder during a suction stroke, and a fuel-air mixture having a homogeneous concentration can be formed in the cylinder. Since air can be utilized with high efficiency in this mode, the amount of injection of the fuel can be increased, and the output of the engine can be sufficiently increased.
In the known in-cylinder type internal combustion engine, as described above, one of the late injection mode and early injection mode is selected as the fuel injection mode, depending upon the current steady-state operating state of the engine, but the fuel injection mode is selected without considering transient operating states of the engine, such as starting, acceleration, deceleration and cold state of the engine. Accordingly, when the engine is in a certain transient operating state, the fuel injection mode or the average air-fuel ratio in the cylinder may not be appropriately selected, and the engine used in the vehicle may not be able to sufficiently perform its desired functions.
In view of the above problem, the late injection mode is forced to be established as the fuel injection mode when an accelerator pedal is depressed during fuel cut so as to resume supply of the fuel and bring the vehicle into an accelerating state, thereby preventing shocks due to acceleration of the vehicle, as disclosed in WO96/36801.
It has also been proposed in laid-open Japanese Patent Publication (kokai) No. 7-279729 to determine whether the fuel injection is resumed in the late injection mode or early injection mode, depending upon a control region selected according to the torque and fuel amount.
Where the fuel injection is resumed in the late injection mode in the in-cylinder injection type engine disclosed in either of the above publications, however, there arises a problem that an undershoot of the engine speed cannot be avoided if the engine speed is reduced by a considerably large degree.
If the engine speed is reduced when the fuel injection mode is forced to return to the late injection mode, only small torque is produced upon return to this mode due to a delay in correction of the amount of air, thus causing a problem of further reduced engine speed.